DESCRIPTION

This command installs a package, and any packages that it depends on. If the package has a shrinkwrap file, the installation of dependencies will be driven by that. See npm help shrinkwrap.

A package is:

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a) a folder containing a program described by a package.json file

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b) a gzipped tarball containing (a)

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c) a url that resolves to (b)

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d) a <name>@<version> that is published on the registry with (c)

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e) a <name>@<tag> that points to (d)

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f) a <name> that has a "latest" tag satisfying (e)

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g) a <git remote url> that resolves to (b)

Even if you never publish your package, you can still get a lot of benefits of using npm if you just want to write a node program (a), and perhaps if you also want to be able to easily install it elsewhere after packing it up into a tarball (b).

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npm install (in package directory, no arguments):

Install the dependencies in the local node_modules folder.

In global mode (ie, with −g or −−global appended to the command), it installs the current package context (ie, the current working directory) as a global package.

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npm install <folder>:

Install a package that is sitting in a folder on the filesystem.

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npm install <tarball file>:

Install a package that is sitting on the filesystem. Note: if you just want to link a dev directory into your npm root, you can do this more easily by using npm link.

Example:

npm install ./package.tgz

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npm install <tarball url>:

Fetch the tarball url, and then install it. In order to distinguish between this and other options, the argument must start with "http://" or "https://"

Example:

npm install https://github.com/indexzero/forever/tarball/v0.5.6

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npm install <name> [−−save|−−save−dev|−−save−optional]:

Do a <name>@<tag> install, where <tag> is the "tag" config. (See npm help config.)

In most cases, this will install the latest version of the module published on npm.

Example:

npm install sax

npm install takes 3 exclusive, optional flags which save or update the package version in your main package.json:

You may combine multiple arguments, and even multiple types of arguments. For example:

npm install sax@">=0.1.0 <0.2.0" bench supervisor

The −−tag argument will apply to all of the specified install targets.

The −−force argument will force npm to fetch remote resources even if a local copy exists on disk.

npm install sax −−force

The −−global argument will cause npm to install the package globally rather than locally. See npm help global.

The −−link argument will cause npm to link global installs into the local space in some cases.

See npm help config. Many of the configuration params have some effect on installation, since that´s most of what npm does.

ALGORITHM

To install a package, npm uses the following algorithm:

install(where, what, family, ancestors)

fetch what, unpack to <where>/node_modules/<what>
for each dep in what.dependencies
resolve dep to precise version
for each dep@version in what.dependencies
not in <where>/node_modules/<what>/node_modules/*
and not in <family>
add precise version deps to <family>
install(<where>/node_modules/<what>, dep, family)

For this package{dep} structure: A{B,C}, B{C}, C{D}, this algorithm produces:

A

+−− B
‘−− C
‘−− D

That is, the dependency from B to C is satisfied by the fact that A already caused C to be installed at a higher level.

See npm help folders for a more detailed description of the specific folder structures that npm creates.

Limitations of npm´s Install Algorithm
There are some very rare and pathological edge−cases where a cycle can cause npm to try to install a never−ending tree of packages. Here is the simplest case:

A −> B −> A´ −> B´ −> A −> B −> A´ −> B´ −> A −> ...

where A is some version of a package, and A´ is a different version of the same package. Because B depends on a different version of A than the one that is already in the tree, it must install a separate copy. The same is true of A´, which must install B´. Because B´ depends on the original version of A, which has been overridden, the cycle falls into infinite regress.

To avoid this situation, npm flat−out refuses to install any name@version that is already present anywhere in the tree of package folder ancestors. A more correct, but more complex, solution would be to symlink the existing version into the new location. If this ever affects a real use−case, it will be investigated.